1. Field of the Invention
The invention relates to inspection systems and, more particularly, to a system for enabling an unobstructed optical or acoustic inspection of physical conditions within a borehole. The invention may be practiced during maintenance and servicing of oil, gas, geothermal, and injection wells.
2. History of the Prior Art
In the drilling and production of oil and gas wells, it is often necessary to obtain at the surface information concerning conditions within the borehole. For example, tools and other objects may become lodged in the borehole during the drilling of a well. Such objects must be retrieved before drilling can continue. When the removal of foreign objects from a borehole is undertaken, known as "fishing", it is highly desirable to know the size and shape of the obstructing object in order to select the proper fishing tool to grasp the object and remove it from the borehole. Such information is very difficult to obtain because of the hostile downhole environment within a borehole filled with opaque drilling fluids.
In the operation and/or periodic maintenance of producing or injection wells, it is also frequently necessary to obtain information about the construction and/or operating condition of production equipment located downhole. For example, detection of the onset of corrosion damage to well tubing or casing within a borehole enables the application of anti-corrosive treatments to the well. Early treatment of corrosive well conditions prevents the highly expensive and dangerous replacement of corrosion damaged well production components. Other maintenance operations in a production well environment, such as replacement of various flow control valves or the inspection of the location and condition of casing perforations, make it highly desirable for an operator located at the surface to obtain accurate, real-time information about downhole conditions. The presence of production fluids in the well renders accurate inspection very difficult.
Various techniques have been proposed for obtaining at the surface information about the conditions within a borehole. One approach has been to lower an acoustic inspection device into the borehole to transmit acoustic energy in the zone of inspection and receive and analyze the reflected acoustic energy as indicia of downhole conditions. Such a system is shown in U.S. Pat. No. 4,766,577 to Clerke et al. A major problem with such acoustic devices is that it is very difficult to obtain a high degree of resolution of details with such acoustic systems. In addition, the presence of non-homogenous borehole fluids produces random scattering of the acoustic energy resulting in even further inaccuracies and distortions of the reflected waveforms.
Television cameras have also been used in prior art downhole inspection systems in an attempt to allow an observer at the surface to obtain an accurate view or recording of physical conditions within a borehole. Such systems are shown in U.S. Pat. Nos. 2,852,600 to Jenkins Jr., 2,912,495 to Moon et al., and 4,229,762 to Healy. A significant drawback inherent in such systems is that the presence of optically opaque borehole fluids obscures the view of such cameras unless the point of observation is directly adjacent the outside of the surface of the lens housing. This is a serious limitation of the applicability of television systems to borehole inspection apparatus.
One attempt to solve this problem with television inspection systems is set forth in U.S. Pat. No. 2,849,530 to Fleet wherein it is proposed pump a slug of clear liquid from the surface down to the point of observation. While this approach is a good attempt to deal with this problem, it is impractical because the difficulty of accurately positioning the slug of clear liquid when the camera is located a substantial distance from the surface. Also, some underground formations may be damaged by fluid injection. Further, the slug of clear liquid will also become mixed with opaque production and/or borehole fluids during its journey from the surface to the downhole location of the camera and thereby be rendered either opaque or translucent by the time it reaches the downhole camera.
It would be a major improvement in downhole inspection systems if a optically clear or acoustically homogenous fluid could be directly injected into the borehole in the zone where inspection is desired. The present invention provides such a system.